If you need gain and good drive, our friend the pentode is there. However, with the high anode resistance, it’s hard to implement as a driver. With a resistor load you get good results, but not optimal. The gyrator load (as a hybrid mu-follower stage) brings a good option to the pentode driver. The workaround to the high gain of the stage has been cleverly addressed by Gary Pimm. Here is just a brief summary of how to implement it:
The circuit can be explained easily. The pentode (U1) is loaded with the gyrator (g1). The pentode screen has a stable voltage (vs) which is provided by the voltage regulator (U2) and the CCS formed by M1+R2. You can implement the screen voltage source that best suits you. Anyhow, the input is provided to the grid (g1) and the grid resistor (Rg) provides ground reference. The cathode resistor (Rk) is un-bypassed. Quite unusual for a pentode. The thing is, we have gain to spare, but thanks to the gyrator, the output impedance of the stage isn’t mu times the Rk. Hence we can afford adding this resistor which also linearise the stage thanks to the negative feedback introduced. Ra is required to provide a stable output and limit the gain. The gain is therefore Gm times the Ra, Gm is degenerated due to Rk (unless you bypass it). Ra could be also be placed in parallel with G1, but as Gary Pimm well explains, it’s better to have it referenced to ground to improve the power supply noise rejection (PSRR).
The output is take from the mu output of the gyrator. The load is connected here. If you need all the gain from this stage you can bypass Rk or better replace Rk with a series of diodes (SiC) or LEDs. Whatever you please.
This stage can be a great driver for a SE stage. Like a 300B. A 4P1L will work brilliantly here. As most of the Russian pentodes.
Also if you want to go further, you can implement a pentode output stage and provide plate to plate feedback (a la Schade) and create a fantastic amp. Michael Koster and Anatoliy have covered this topology at length in DIYaudio, check it out. If you elevate the cathode of the output stage you can DC-couple it. Great stuff and sounds amazing, I did implement this with my 814 SE Amp.
As you can see, a very flexible stage, thanks to the gyrator. Once again, a very handy topology to use.
Cheers
Ale
